Jeffrey S. Pontius

Carbon and nitrogen stoichiometry and nitrogen cycling rates in streams

Stoichiometric analyses can be used to investigate the linkages between N and C cycles and how these linkages influence biogeochemistry at many scales, from components of individual ecosystems up to the biosphere. N-specific NH4+ uptake rates were measured in eight streams using short-term 15N tracer additions, and C to N ratios (C:N) were determined from living and non-living organic matter collected from ten streams. These data were also compared to previously published data compiled from studies of lakes, ponds, wetlands, forests, and tundra. There was a significant negative relationship between C:N and N-specific uptake rate; C:N could account for 41% of the variance in N-specific uptake rate across all streams, and the relationship held in five of eight streams. Most of the variation in N-specific uptake rate was contributed by detrital and primary producer compartments with large values of C:N and small values for N-specific uptake rate. In streams, particulate materials are not as likely to move downstream as dissolved N, so if N is cycling in a particulate compartment, N retention is likely to be greater. Together, these data suggest that N retention may depend in part on C:N of living and non-living organic matter in streams. Factors that alter C:N of stream ecosystem compartments, such as removal of riparian vegetation or N fertilization, may influence the amount of retention attributed to these ecosystem compartments by causing shifts in stoichiometry. Our analysis suggests that C:N of ecosystem compartments can be used to link N-cycling models across streams.

Evaluation of a reproductive index to estimate dickcissel reproductive success

Intensive monitoring of bird nests to measure reproductive success is time-consuming and may influence the fate of nests. Reproductive indices that do not require searching for and visiting nests may be reasonable alternatives to nest monitoring if they provide results similar to nest-searching efforts. We evaluated the reproductive index of Vickery et al. (1992) for estimating reproductive success of the dickcissel (Spiza americana) in northeast Kansas, USA. We used nest searching and Vickery et al.s (1992) reproductive index to compare reproductive success on 20 plots (200 x 200 m). Daily nest survival (DNS) rates averaged 0.911 (SE = 0.011, n = 72 nests), and brown-headed cowbirds (Molothrus ater) accounted for 21% of all nest failures. Surveyors underestimated reproductive index ranks when compared to nest-searching efforts and were inaccurate in their assignment of reproductive success. In particular, surveyors reported successful nests on 3 study plots that fledged no young, probably because young dickcissels moved onto plots after fledging from their natal territories. Our results indicate that the reproductive index of Vickery et al. (1992) may be inappropriate for wary species or those heavily parasitized by brood parasites. We suggest that before relying on the index alone, investigators should use pilot trials to determine whether results from this index are concordant with results from intensive nest-searching efforts for species of interest.

The Effects of Bison Crossings on the Macroinvertebrate Community in a Tallgrass Prairie Stream

Abstract The distribution and diversity of macroinvertebrates in relation to bison crossings was studied for four seasons in a headwater reach of a tallgrass prairie stream. Species richness was significantly higher above crossings only during the fall. Summer samples revealed significantly higher EPT (richness within only Ephemeroptera, Plecoptera and Trichoptera) index values above crossings than at crossings. On average, over 10% more fine sediment (<0.2 mm) occurred at the crossings than above or below crossings. Densities of two taxa (Hydropsyche spp. and Elmidae) were significantly higher above or below crossings, whereas three taxa (Bezzia/Palpomyia spp., Culicoides sp., and Naididae/Tubificidae) were significantly higher at crossings for at least one season. Overall, this study suggests that impact of bison on benthic communities of headwater streams was spatially limited. Although bison may have less impact upon stream communities than previous studies of cattle impacts have shown, differences in climate and stocking rate could be important factors.

Plant community and bare ground trends on Fort Riley, Kansas: Implications for monitoring of a highly disturbed landscape

Abstract The Land Condition Trend Analysis (LCTA) program was developed by the US Army and implemented on Fort Riley, Kansas, to monitor trends in plant communities so that managers could maintain quality training lands without the loss of ecological diversity. As a standard protocol, vegetation surveys using the point-intercept method were conducted from 1994–2001. Presence of bare ground, soil erosion, and military vehicle traffic were also recorded. At the community level, species richness usually fluctuated < 10% annually and declined 6% from 1994–2001. Species richness per plot increased from 1994 to 2001 by 6.5% as compared to the 1994 estimate. Annual species richness of noxious plants ranged from 18–26 species; noxious species per plot averaged 1.0–2.4 species annually with higher estimates usually observed in grassland (vs. woodland) habitats. Modest year-to-year changes were noted at the species level for the most frequently observed species based on an index of relative difference. Two noxious plant species, sericea lespedeza (Lespedeza cuneata) and Johnsongrass (Sorghum halepense), first detected during the second year of monitoring, expanded their distribution considerably on the fort. This expansion may have resulted when mechanized training facilitated transport of seed on vehicles from one training area to another. The patterns of expansion (both on a plot and on a training area basis) by these two noxious species revealed that our analysis approach may be useful for detecting early stages of infestations on this highly disturbed landscape. Bare ground conditions increased on an average of 1.5% per year, but may not be completely attributable to military training activities. Because there appears to be little year-to-year fluctuation in the diversity of the plant communities and trends for most individual plant species monitored on Fort Riley, we recommend detailed plant surveys not be conducted annually. An exception would be monitoring of noxious plant species with a method other than the point-intercept technique, which is extremely time-consuming.

Influence of Time Interval on Estimations of Movement and Habitat Use

Abstract Although biotelemetry has been widely used in fisheries science, the standardization of methods is uncommon. Researchers often use more than one time interval (frequency of recording locations) in a study, and different researchers frequently use different time intervals. There is a paucity of information describing the relationship between time interval used and movement observed or the proportion of time associated with habitat type. The objectives of this study were to evaluate the effects of varying time intervals on fine-scale (i.e., minutes-to-hours) diel movement and habitat use by spotted bass Micropterus punctulatus. To evaluate such effects, we tracked spotted bass (n = 11) every 15 min for a 24-h period. Using these data we simulated tracking at 30-min, 1-h, and 2-h intervals. The mean percent error of time interval simulations for total daily movement varied from 24.3% to 64.3% for 30-min and 2-h intervals, respectively. Loss of movement information (i.e., detecting less movement with...

Strip Adaptive Cluster Sampling: Probability Proportional to Size Selection of Primary Units

SUMMARY Strip adaptive cluster sampling is extended to situations where the primary units have unequal numbers of secondary units. Primary units are selected with replacement and with probabilities proportional to the number of secondary units. The Horvitz-Thompson estimator of the parametric total is used as the basis for estimation. Two cases are considered: basic strip adaptive cluster sampling and strip adaptive cluster sampling under stratification of primary units. A numerical example is presented, in which the goal is to estimate the number of objects on a site. The sampling strategy is useful for ecological studies where a rectangular grid of secondary units is not adequate to cover the study site.

Short-Term Effects of Cattle Exclusion on Riparian Vegetation in Southeastern Kansas

Abstract Effects of cattle exclusion on the structure and composition of riparian vegetation were observed in a 2-yr study in southeastern Kansas. The study was conducted within riparian habitats on the 5,263-ha Kansas Army Ammunition Plant in north-central Labette County, Kansas. Three grazed and three ungrazed riparian areas were sampled in 1996 and 1997 to monitor vegetation changes in response to livestock exclusion. Total understory, grass, and litter cover were significantly different between the grazed and ungrazed study sites with mean cover estimates being higher (16.3%, 14%, and 12.1% greater respectively) in the ungrazed sites. A significant difference in the percentage of bare ground was observed between the grazed (24.6%) and ungrazed (12.5%) study sites. No difference in herbaceous vegetation height was detected between study sites in 1996. In 1997, mean herbaceous vegetation height differed significantly from 1996 (study sites combined) and was greater (95.6 cm vs. 65.6 cm) in the ungrazed study sites. Excluding cattle from closed canopy riparian woodlands in southeastern Kansas resulted in a positive short-term response of understory herbaceous vegetation. Our results suggest that riparian fencing may be an effective management tool for restoring understory vegetation in riparian communities grazed by cattle in the eastern Great Plains.

Breeding season avifauna of prairie dog colonies and non-colonized areas in shortgrass prairie

Abstract Avian assemblages of black-tailed prairie dog (Cynomys ludovicianus) colonies were compared to those present at two types of non-colonized grassland, during the breeding season, in southwestern Kansas and southeastern Colorado. Relative abundances were quantified in 1996 during a period of drought and in 1997 during a period of above average precipitation. We detected fewer bird species at all sites in 1996 than in 1997, and the total number of bird species detected on prairie dog colonies was lower than that detected on both types of non-colonized areas during both years. Horned larks (Eremophila alpestris) had higher relative abundances on non-colonized sites in 1996, but were more abundant on prairie dog colonies in 1997. Lark buntings (Calamospiza melanocorys) and grasshopper sparrows (Ammodramus savannarum) were not recorded at the study sites in 1996, but appeared in high numbers in 1997. These two species, and western meadowlarks (Sturnella neglecta), had higher relative abundances on non-colonized sites in 1997 than on prairie dog colonies. A comparison of our results with those of other studies suggest that the effects of prairie dogs on associated biota might not be identical in all regions of the Great Plains, or under all environmental conditions.

Estimation of the mean in line intercept sampling

Ratio estimation of the parametric mean for a characteristic measured on plants sampled by a line intercept method is presented and evaluated via simulation using different plant dispersion patterns (Poisson, regular cluster, and Poisson cluster), plant width variances, and numbers of lines. The results indicate that on average the estimates are close to the parametric mean under all three dispersion patterns. Given a fixed number of lines, variability of the estimates is similar across dispersion patterns with variability under the Poisson pattern slightly smaller than varia-bility under the cluster patterns. No variance estimates were negative under the Poisson pattern, but some estimates were negative under the cluster patterns for smaller numbers of lines. Variance estimates become closer to zero similarly for all spatial patterns as the number of lines increases. Ratio estimation of the parametric mean in line intercept sampling works better, from the viewpoint of approximate unbiasedness and variability of estimates, under the Poisson pattern with larger numbers of lines than other combinations of spatial patterns, plant width variances and numbers of lines.

Conventional bootstrap and normal confidence interval estimation under adaptive cluster sampling

We examine the performances of bootstrap and normal confidence interval methods for estimating the parametric total when using the Horvitz–Thompson type estimator under adaptive cluster sampling via simulation. On the basis of this and previous work, we include recommendations on using confidence interval methods under adaptive cluster sampling. Two Poisson spatially clustered populations and three sample sizes were used in our simulations. The bootstrap methods (Gross, McCarthy and Snowden, and Sitter) were based on resampling the original sample of units. All four interval estimation methods performed poorly. On average, bootstrap confidence interval estimates underestimated the parametric totals, resulting in empirical coverages lower than the set confidence level. Coverages for the normal method depended on initial sample sizes and population. In general, if confidence interval estimates are required under adaptive cluster sampling, we recommend using bootstrap interval estimation with the Hansen–Hurwitz type estimator instead of interval estimation with the Horvitz–Thompson type estimator.